The present invention relates to a vehicle brake control unit and particularly relates to an antilock brake (ABS) control unit.
An ABS performs control that judges a wheel lock tendency, decreases the pressure inside a wheel cylinder to release a lock state when lock is detected, and gradually increases the pressure inside the wheel cylinder to strengthen the brake when the lock state is released. Consequently, in ABS control, if the pressure decrease in the wheel cylinder after a lock state of the wheel has been detected is insufficient, the lock state cannot be sufficiently eliminated. Further, if the pressure increase in the wheel cylinder after a lock state of the wheel has been released, the vehicle cannot be sufficiently decelerated. Therefore, in ABS control, it is desired to avoid an insufficient pressure decrease in the wheel cylinder after a lock state has been detected and an insufficient pressure increase in the wheel cylinder after a lock state has been released.
In ABS control, generally, a wheel lock tendency is judged by using a wheel speed sensor to calculate the slip rate of the wheel. The amount of pressure decrease and the amount of pressure increase at the time of ABS actuation are mainly decided on the basis of the result of measurement by the wheel speed sensor. There exist various ABS control algorithms for shortening the braking distance without compromising the stability of the vehicle. For example, there is a control scheme that estimates the friction coefficient μ of the road surface from the measurement value of the wheel speed sensor, decides a target amount of pressure decrease and a target amount of pressure increase in accordance with the estimated low μ or high μ road surface state, and drives and controls an electromagnetic valve so as to realize the decided amount of pressure decrease and amount of pressure increase. Alternatively, there is also a control scheme that uses a hydraulic pressure sensor when deciding the target amount of pressure decrease and the target amount of pressure increase (e.g., JP-A-2006-176046).
Generally in conventional ABS control, when the target amount of pressure decrease and the target amount of pressure increase to be applied to the wheel cylinder are decided, thereafter pressure control of the wheel cylinder is performed by an electromagnetic valve or the like so as to achieve the target amount of pressure change. However, even in the case of using a hydraulic pressure value that has been measured by a hydraulic pressure sensor for deciding the target amount of pressure change in the wheel cylinder, whether or not the pressure in the wheel cylinder has achieved the target value by the driving of the electromagnetic valve is not confirmed.
Consequently, it is not always the case that the actual pressure inside the wheel cylinder has become the decided target during the operation of the ABS, and due to the effects of the operating environment and so forth, depending on the case the amount of pressure increase and the amount of pressure decrease in the wheel cylinder at the time of ABS operation may potentially deviate from the proper value or range. As a result, the braking distance may potentially become longer in a case where the amount of pressure increase is insufficient, and the stability of the vehicle may potentially be compromised in a case where the amount of pressure decrease is insufficient.
Further, in ABS control of recent years, functions are becoming increasingly sophisticated, and sometimes complicated control logic is used, for example, to discriminate the road surface state and the vehicle state and change control parameters in accordance with those states. However, it is difficult to evaluate and test combinations of all states under every situation. For that reason, pressure control of the wheel cylinder may potentially be performed outside the proper range of change in the pressure in the wheel cylinder that is ordinarily conceivable. For example, the amount of pressure increase in the wheel cylinder may potentially become smaller than assumed in a case where a function that suppresses the pressure increase gradient of the pressure in the wheel cylinder at the time of ABS actuation as a result of discriminating the road surface state and a function that suppresses the pressure increase gradient of the pressure in the wheel cylinder at the time of ABS actuation as a result of discriminating the vehicle state have both acted. In such a case, the problem that the braking distance becomes longer than assumed may potentially arise.
Therefore, in a vehicle brake control device, it is desired to keep the actual amount of pressure increase and the actual amount of pressure decrease inside the wheel cylinder at the time of ABS control in the proper range even in the case of performing ABS control on the basis of any control logic.